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# Rate of reaction when cutting y-axis watch

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2. (Original post by Kreayshawn)
I've plotted a graph of concentration against time

The results are straight for the first few points then the last few start to level out

It's a positive gradient ( / shape )

I need to calculate the rate of reaction - I understand this is the gradient of the graph, however, I've had to cut the y-axis in order to draw the graph.

My first result is at 120 seconds and if I try drawing the graph with the line from the origin

Can I calculate the overall rate of the reaction without drawing the initial rate from the ORIGIN? can I just draw the tangent from where the curve starts to level off?

These quick drawings I've done might help explain what I'm trying to ask, I'm having a difficult time trying to explain it:

when you think about kinetics, you should have started the stopwatch(t=0) when you bring the reactants to react. At this point, t = 0, rate would be considered to have just started, from where is the question; from zero (i.e. there is no reaction prior to t =0 - you catch my drift here?)

then you'd get a curve and eventually a plateauing effect due to reactant(s) fully used up. i'd go for your second graph. the tangent/gradient is how books always show anyway - does your book represent these differently?
3. (Original post by Kreayshawn)

Appreciate any help - sorry for making it very wordy. Hopefully I got my ideas across properly
Without over-complicating things, let me just ask you, how do you define the time for your graph? i.e. what is it for, details. perhaps that will clarify why there is no reading until you reach like 120 s.
4. (Original post by shengoc)
Without over-complicating things, let me just ask you, how do you define the time for your graph? i.e. what is it for, details. perhaps that will clarify why there is no reading until you reach like 120 s.
I started a reaction, quenched it at specific time periods (2, 5, 10, 15, 20, 25 minutes), so I took solution out using a volumetric pipette and put it into a solution in a conical flask so it stopped reacting then titrated it to find the current concentration at each interval, then plotted concentration against time
5. (Original post by Kreayshawn)
I started a reaction, quenched it at specific time periods (2, 5, 10, 15, 20, 25 minutes), so I took solution out using a volumetric pipette and put it into a solution in a conical flask so it stopped reacting then titrated it to find the current concentration at each interval, then plotted concentration against time
so actually your reaction won't start at t = 0, before you mix reactants together. As far as i understand, when you start timing from the moment you mix the reactants, then rate = 0 at t = 0.

then as you quenched at particular time, in your case, you decide to take the first reading at t = 2 minutes; if you have gotten reading at t = 1 minute, great, but if not, you know it'd lie in between that for t = 0 and t = 2. you get me?

so i'd say draw from x=0, y=0 to your first point, then join up the rest of the curve.

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Updated: March 31, 2013
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